Liquid-crystal-on-silicon (LCOS) has gained considerable attention due to recent innovations in reflection projection technologies. FIG. 1 shows a cross-sectional view of a conventional LCOS structure 100. The LCOS structure comprises a substrate 105. Variety of devices such as transistors (not shown) can be formed on the substrate. A dielectric layer, for example, an intermetal dielectric (IMD) layer 120 is formed on top of the substrate. A patterned adhesive layer 130, such as Ti or TiN, and a patterned metal layer 160 are formed over the substrate. An opening, such as a recess or trench, is formed in the adhesive and metal layers, exposing portion of the IMD layer. A passivation oxide layer 170 and a passivation nitride layer 180 are deposited over the substrate and the opening. Non-planar passivation layers are thus formed. Liquid crystals (not shown) are then applied directly to the surface of the semiconductor device coated with metal layer and non-planar passivation layers. As such, high reflectance of the metal layer is compromised.
Flatness of the metal layer is one of the major factors affecting the reflectivity in LCOS devices. To obtain optimal performance, besides requiring high reflectivity of the metal layer, planarization of the passivation layers over the metal layer is important too.
From the foregoing discussion, it is desirable to provide planarized passivation layer for LCOS devices.